1. Field of the Invention
The present invention relates to a method for fabricating a complementary metal-oxide-semiconductor (CMOS) image sensor, and more particularly, to a method for fabricating a microlens of a CMOS image sensor.
2. Discussion of the Related Art
FIG. 1A illustrates a related art method for fabricating a CMOS image sensor. As shown, a metal line 1a is formed on a sublayer 1. Then, an etching process is performed to form a pad opening 1b, in which an aluminum pad 2a is formed. A pad layer 2 is formed on the sublayer 1, a color filter array 3 is formed on the pad layer 2, and a planarization layer 4 is formed on the color filter array 3. Then, a reflow process for the formation of a microlens 6 is performed. The reflow process includes forming and patterning a photoresist 5 on the planarization layer 4 and then reflowing the patterned photoresist 5 to form the structure shown in FIG. 1B. By this process of reflowing the patterned photoresist 5, the microlens 6 obtains its desired structure including the characteristics of the curvature of its upper surface, its overall size, its configuration, etc.
In the related art, the aluminum pad 2a is contaminated during a photolithography process for patterning an aluminum layer. That is, the patterned aluminum is a particle source that generates particles which remain on the pad 2a. 
The formation of the color filter array 3 and the microlens 6, including the reflowing of the photoresist 5, are the final steps of a fabrication process of the related art CMOS image sensor. However, a precise shape of the curved surface of the microlens 6 is difficult to obtain. For example, for a smaller microlens, the curved surface requires a smaller radius and a shorter vertical dimension or peak height of the microlens. The structure of a smaller microlens needs to be shallower to enable an increase in packing density, but it has been particularly difficult to form a microlens having such a lower height as required.